The invention relates to a compressor system for a vehicle, having a compressor driven by a drive motor of the vehicle, and a suction air conduit for supplying air that has already been precompressed by a turbocharger of the drive motor to the compressor.
The invention furthermore relates to a method for operating a compressor system for a vehicle, having a compressor driven by a drive motor of the vehicle, and a suction air conduit for supplying air that has already been precompressed by a turbocharger of the drive motor to the compressor.
Vehicles with air-operated component systems, e.g. a pneumatic suspension or air brakes, generally have a compressor driven by a drive motor of the vehicle for the purpose of producing the required compressed air. To increase the energy efficiency of the vehicle, the drive motor used is often fitted with a turbocharger. The compressor driven by the drive motor is generally designed as a compressor which draws in ambient air. The performance of the compressor can be significantly enhanced if it is also pressure-charged. This can be accomplished, for example, by drawing in already precompressed air downstream of a compressor of the turbocharger and of an associated charge air cooler. A disproportionate increase in the air volume delivered and better efficiency is thereby achieved through a reduction in flow losses in the intake system of the compressor. Another advantage is the reduction in oil consumption, which essentially follows from the fact that there is no vacuum in the cylinder during an air intake phase, in contrast to a conventional compressor, which draws in ambient air.
However, one disadvantage of a pressure-charged compressor is that, in blow-off mode, i.e. in an idling phase, in which the compressor is not delivering any compressed air into the supply system of the vehicle, it continues to deliver a large volume of air, and this results in a significantly higher power loss in this operating condition than is the case with a conventional compressor, which draws in ambient air.
It is therefore the underlying object of the invention to eliminate this disadvantage, at least in part, while at the same time ensuring that the oil consumption of the compressor remains at a low level.
This and other objects are achieved by a compressor system for a vehicle, having a compressor driven by a drive motor of the vehicle, and a suction air conduit for supplying air that has already been precompressed by a turbocharger of the drive motor to the compressor. A mechanism for reducing the flow cross section is disposed in the suction air conduit. The mechanism is used to limit the charging pressure of the already precompressed air supplied to the compressor.
By disposing the mechanism for reducing the flow cross section in the suction air conduit, it is possible, by way of the backpressure produced, to achieve any desired reduction in the charging pressure supplied to the pressure-charged compressor. In particular, the charging pressure supplied to the compressor can be reduced in a blow-off or idling phase, thus reducing the volume of air delivered in the idling phase, which reduces the power consumption of the compressor. Examples of mechanisms that can be used for reducing the flow cross section are a throttle flap, a slide, which can be pushed into the suction air conduit perpendicularly to the direction of air flow, or a rotatably mounted ball pierced by a hole, of the kind known from a ball valve, for example.
It is advantageous here if provision is made for the mechanism for reducing the flow cross section to be actuable mechanically by a pneumatic working cylinder. The use of a pneumatic working cylinder for actuating the mechanism for reducing the flow cross section allows rapid and precise adaptation of the exposed flow cross section of the suction air conduit and hence, by way of the adaptation of the backpressure caused, adaptation of the charging pressure provided for the compressor.
It is particularly preferred that the working cylinder comprises a mobile piston. The use of a working cylinder with a mobile piston makes it possible to construct an extremely robust pneumatically driven actuating mechanism with a lever travel of any desired length which can be defined by the length of the working cylinder.
As an alternative, however, it is also possible to make provision for the working cylinder to comprise a flexible diaphragm. The use of a flexible diaphragm instead of a mobile piston makes possible, in particular, shorter response times for the working cylinder since the mass inertia to be overcome is lower. This is advantageous particularly in the case of short idling phases.
Ideally, provision is made to enable the pneumatic working cylinder to be supplied via a pneumatic control line with the charging pressure prevailing downstream of the mechanism for reducing the flow cross section. Supplying the working cylinder directly with the charging pressure prevailing downstream of the mechanism for reducing the flow cross section allows self-adaptation of the charging pressure provided and the flow cross section exposed. If the charging pressure provided is greater than that allowed, the exposed flow cross section is reduced by activating the working cylinder by way of a mechanism, as a result of which, in turn, the charging pressure provided is reduced. Conversely, if the charging pressure provided is too low, the working cylinder is deflected to a lesser extent, thereby enlarging the exposed flow cross section, which results in a higher charging pressure being provided. However, the maximum value of the charging pressure provided cannot exceed the charging pressure prevailing upstream of the mechanism for reducing the flow cross section.
Provision can be made to enable the pneumatic working cylinder to be supplied with a working pressure via a relay valve, it being possible for a pneumatic control input of the relay valve to be supplied via a pneumatic control line with the charging pressure prevailing downstream of the mechanism for reducing the flow cross section. The use of a relay valve enables the working cylinder to be activated with a higher pressure level, exhibiting larger pressure fluctuations, than the pressure fluctuations downstream of the mechanism for reducing the flow cross section. As a result, the working cylinder can be activated more precisely.
It is expedient if provision is made for a 3/2-way valve with its own vent to be disposed in the pneumatic control line to enable the control line to be set to a depressurized condition. Providing a 3/2-way valve with its own vent in the pneumatic control line makes it possible to prevent actuation of the working cylinder. When the pneumatic control line is set to a depressurized condition, a maximum possible charging pressure is provided for the compressor since the mechanism for reducing the flow cross section disposed in the suction air conduit then produce the minimum possible backpressure.
One advantageous possibility is to provide for the compressor system to include an electrically activatable continuously variable valve with a pneumatic supply pressure input, an electric control input and a pneumatic output for actuating the working cylinder, for the charging pressure prevailing downstream of the mechanism for reducing the flow cross section to be detectable by a pressure sensor, and for the compressor system to include an electronic control unit which is suitable for producing an electric control signal for actuating the continuously variable valve as a function of the charging pressure determined in order to limit the charging pressure. The use of an electrically activatable continuously variable valve, a term denoting a directional control valve which does not operate in discrete steps but allows a continuous transition between the control positions, in combination with the pressure sensor disposed downstream of the flow cross section reduction mechanism allows precise actuation of the flow cross section reduction mechanism. In particular, the continuously variable valve can be embodied as a proportional valve with a nonlinear volume flow characteristic, as a control valve with a linear volume flow characteristic or as a servo valve, i.e. a directional control valve with analog activation capability. The use of a pressure sensor avoids unconditioned air precompressed by the turbocharger being used for direct or indirect actuation of the mechanism for reducing the flow cross section. In particular, since the air precompressed by the turbocharger has not yet been dried, the pneumatic devices used to actuate the flow cross section reduction mechanism are protected in an effective manner from corrosion.
In this context, it is also contemplated for the mechanism for reducing the flow cross section to be actuable by an electrically activatable servomotor. Using an electrically activatable servomotor likewise makes it possible to avoid corrosion-induced failure of the pneumatic/mechanical actuating device.
It may furthermore be useful for the compressor system to include an electric control unit which is suitable for producing an electric control signal for activating the electrically activatable servomotor in order to limit the charging pressure, the control signal being based on a charging pressure determined by a pressure sensor downstream of the mechanism for reducing the flow cross section. Using an electronic control unit together with a servomotor to actuate the mechanism for reducing the flow cross section allows flexible adaptation of the charging pressure supplied to the compressor.
Provision can furthermore be made for the compressor system to include a clutch assigned to the compressor, which is suitable for decoupling the compressor completely from the drive motor. Using a clutch to decouple the compressor completely from the drive motor makes it possible to reduce to zero both the oil discharge and the energy consumption of the compressor in an idling phase.
The method of the invention is such that the charging pressure of the already precompressed air supplied to the compressor is limited by an actuating mechanism for reducing the flow cross section to an adjustable maximum value, which mechanism is disposed in the suction air conduit. In this way, the advantages and special features of the compressor system according to the invention are also exploited in the context of a method for operating a compressor system.
This also applies to the particularly preferred embodiments of the method according to the invention which are given below.
The method is developed further in an expedient manner if the maximum value chosen varies as a function of the operating condition. Adapting the maximum value of the charging air supplied to the compressor to the operating condition of the compressor system allows optimization of the operating behavior of the compressor system. It is possible, for example, in an idling phase, to reduce the energy consumption of the compressor by lowering the charging pressure provided, while a maximum possible charging pressure is provided for the compressor in a delivery phase of the compressor system at a low compressor speed in order to maximize the air volume delivered. Moreover, it is likewise possible to specify a maximum permissible charging pressure of the compressor in a delivery phase of the compressor system in order, in particular, to limit the thermal stress on the compressor.
It is advantageous to make provision for the maximum value in an idling phase to be chosen as a function of at least one of the following variables: oil ejected by the compressor, power loss of the compressor. In order to minimize the power loss of the compressor in an idling phase, the charging pressure provided for the compressor should be as low as possible. The charging pressure provided should ideally correspond to ambient pressure, as a result of which the power loss of the compressor would be identical with the power loss of a compressor that draws in ambient air. At the same time, however, the compressor should eject as little oil as possible, with the amount of oil ejected by the compressor rising as the charging pressure falls, owing to the increasing vacuum in the piston space during a suction phase. It is therefore advantageous to determine the maximum permissible charging pressure as a function of the permissible power loss in an idling phase and the permissible oil ejection in an idling phase.
It is particularly preferred that, when the adjustable maximum value is exceeded, the charging pressure supplied to the compressor be held constant, irrespective of the compressor speed and the charging pressure provided by the turbocharger, by dynamic adaptation of the flow cross section exposed by the mechanism for reducing the flow cross section. The dynamic adaptation of the charging pressure supplied to the compressor, in an idling phase for example, keeps the amount of oil ejected by the compressor, in particular, to a constantly low level.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.